The invention relates to a polyaxial bone anchoring system intended for correcting and stabilizing vertebrae of a spinal column.
The invention more particularly relates to a bottom-loading polyaxial bone anchoring system.
Conventionally, the vertebral anchoring systems comprise bone anchoring elements, such as screws, respectively comprising a threaded rod intended to be fixed in the pedicle of a vertebra and provided at one of its ends with a connector for coupling the bone anchor element with a stabilizing element, such as a connecting rod.
In complex assemblies comprising several spinal segments, bone anchoring systems hinged at the junction of the bone anchoring elements and the connectors are commonly used. This is called a polyaxial bone anchoring system or a polyaxial pedicle screw. Adjusting the position of the bone anchoring systems and thus facilitating the mounting of the connecting rods into the latter thus becomes possible.
In order to provide greater modularity as regards the type and the dimensions of the bone anchoring elements and the connectors to be assembled, bone anchoring systems enabling the bottom-loading of the bone anchoring elements with the connectors have been developed.
Among this type of bone anchoring system, the one described in patent U.S. Pat. No. 7,947,065 can be mentioned. The bone anchoring system described comprises a connecting assembly comprising a connector consisting of an upper connecting member and a lower connecting member, an upper retaining ring accommodated in the lower part of the upper connecting member and a split retaining ring accommodated in the lower connecting member of the connector. The lower surface of the upper connecting member has a plurality of ramps so arranged as to cooperate with the lower split ring when assembling the screw with the connector. These ramps make it possible to open the split ring so as to let the spherical head of the screw through the split ring. When the connection assembly is mounted on the screw head, the split ring is thus pushed by the screw head to come into contact with the upper retaining ring which compresses a spring to cause the contacting of the lower split ring with the ramps, thus causing opening of the split ring to allow the passage of the screw head through said ring. Once the screw head is gone therethrough, the spring drives the split ring to the lower part of the lower connecting member, thus preventing the screw head from going out of the connecting assembly.
The connecting system mentioned in patent U.S. Pat. No. 7,947,065, and more generally the bottom-loading bone anchoring systems of the prior art, has/have the disadvantage of being relatively complex. As a matter of fact, the implementation of several elements is required for the screw head to be inserted into and retained in the connector, such as a spring, two retaining means and a ramp system.
The invention aims at remedying these problems by providing a bone anchoring system enabling the bottom-loading of the bone anchoring element into a connector in a simple and fast way while providing a satisfactory behavior of the bone anchoring element in the connector.